Question: The scale on a map is 4cm : 10km. If two cities are 40cm apart on the map, what is the actual distance in km between the two cities?
Solution: The scale means that every 4cm on the map corresponds to an actual distance of 10km. A distance of 40cm on the map is the same as 10 $\cdot$ 4cm. The actual distance between the two cities is 10 $\cdot$ 10km, or 100km.